netdev - Re: [PATCH bpf 0/4] bpf: remove __rcu annotations from bpf_prog

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Date:   Tue, 14 May 2019 19:11:44 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf 0/4] bpf: remove __rcu annotations from bpf_prog_array

On 05/14, Alexei Starovoitov wrote:
> On Tue, May 14, 2019 at 10:53 AM Stanislav Fomichev <sdf@...ichev.me> wrote:
> >
> > Existing __rcu annotations don't add anything to the safety.
> 
> what do you mean?
> BPF_PROG_RUN_ARRAY derefs these pointers under rcu.
And I'm not removing them from the struct definitions, I'm removing __rcu
from the helpers' arguments only. Because those helpers are always called
with the mutex and don't need it. To reiterate: rcu_dereference_protected
is enough to get a pointer (from __rcu annotated) for the duration
of the mutex, helpers can operate on the non-annotated (dereferenced) prog
array.

Read section still does the following (BPF_PROG_RUN_ARRAY):

	rcu_read_lock();
	p = rcu_dereference(__rcu'd progs);
	while (p) {}
	rcu_read_unlock();

And write sections do:

	mutex_lock(&mtx);
	p = rcu_dereference_protected(__rcu'd progs, lockdep_is_held(&mtx);
	// ^^^ does rcu_dereference in the mutex protected section
	bpf_prog_array_length(p);
	bpf_prog_array_copy_to_user(p, ...);
	bpf_prog_array_delete_safe(p);
	bpf_prog_array_copy_info(p);
	bpf_prog_array_copy(p, ...);
	bpf_prog_array_free(p);
	// ^^^ all these helpers are consistent already with or
	// without __rcu annotation because we hold a mutex and
	// guarantee no concurrent updates, so __rcu annotations
	// for their input arguments is not needed.
	mutex_unlock(&mtx);